The invention relates to the preparation of long-chain inulin from plant extracts while simultaneously obtaining glucose and fructose.
Inulin is a polysaccharide belonging to the group of the fructans. Economically obtainable amounts are found in a series of plants, such as Jerusalem artichoke tubers and dahlia tubers, and also in chicory roots. It is a heterofructan since a .beta.-1,2-linked chain of fructose molecules is terminated by an .alpha.-D-glucose at the reducing end. The chain length depends both on the plant species and on the growth stage of the plant.
It is known (F. Perschak and L. Wolfslehner, Zuckerind., 115 (1990) 466-470) to extract inulin from the plant tissue using hot water, to remove the salts from the extract, to decolorize it and to obtain inulin from the product in dry form. This inulin, which contains monosaccharides, such as glucose and fructose, and disaccharides, such as sucrose and fructo-oligosaccharides in addition to the polymeric component, is used as a raw material for the preparation according to the invention of long-chain inulin. It will be termed "crude inulin" hereinbelow.
In contrast, DE-A 4,003,140 discloses the enzymatic cleavage of inulins to give shorter chains and the use of the fraction of a degree of polymerization of approximately 3-7 as calorie-reduced sugar substitutes which are suitable for diabetics.
Because of its mono- and disaccharide content, the crude inulin is unsuitable for the preparation of dietetic foodstuffs, in particular those intended for diabetics. Also, due to the hygroscopicity and tackiness of the short-chain oligosaccharides, they can be very troublesome when crude inulin is used in foodstuffs both in processing and in storing.
The sweet taste of the short-chain oligosaccharides in the crude inulin is troublesome for a series of applications in the food sector (for example meat products). Since long-chain inulin has a neutral flavor, it is better suited for such uses than crude inulin.
Due to the mono- di- and oligosaccharide content of crude inulin, its chemical or biochemical conversion also only gives undefined product mixtures whose purification is virtually impossible.
It is therefore desirable to obtain, by removal of the short-chain oligosaccharides, an inulin product from the crude inulin which has only long-chain molecules. Short-chain oligosaccharides are also to be understood as including molecules which have degrees of polymerization (DP) of up to 10-12. Accordingly, long-chain inulin is virtually free from the molecules of DP&lt;10-12 and therefore has, in the case of chicory inulin, an average chain length of &gt;20.
A series of processes is suitable for the preparation of such a long-chain inulin.
Long-chain inulin can be precipitated under suitable conditions from an aqueous solution in the presence of high concentrations of organic solvents, such as methanol, ethanol, isopropanol or their mixtures and then isolated using a centrifuge or pressure filter. On the other hand, the disadvantage of this process is that it involves organic solvents whose use is especially problematic, particularly in the food sector. On the other hand, the process involves large volumes, which means that the dissolved components which have been separated off, such as glucose, fructose, sucrose and oligosaccharides have to be recovered from dilute solutions and the yield of the desired long-chain inulins is low due to losses caused by dissolution.
Aqueous inulin solutions can also be subjected directly to crystallization with an addition of seed crystals, so that mainly long-chain inulins are precipitated and can be removed by centrifugation. However, the resulting products are relatively badly contaminated with the accompanying substances, in particular the short-chain inulins. In this process, too, a large proportion of the higher inulins remain in the mother liquor and are thus lost.
BE 92/010921 (unpublished) discloses the removal of low-molecular-weight components of up to approximately DP 5 by means of chromatographic separation of the inulin. In this case, the resulting inulin is contaminated with substantial amounts of short-chain molecules due to the poor separating effect and thus has a low mean chain length.
Application WO 91/18000 describes a process for obtaining oligosaccharides from biomass by means of ultrafiltration. However, the use of this process for the separation of inulin involves some serious disadvantages. The mono- and disaccharides and short-chain oligosaccharides are obtained in the permeate in extremely dilute solutions and can only be recovered with a high input of energy. The membranes are expensive and furthermore only have short working lives. Moreover, they tend to foul readily.
It was therefore an object to find a novel process for the preparation of long-chain inulins which avoids the disadvantages of the known processes.